1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal composition and a ferroelectric liquid crystal device containing the same.
2. Description of the Related Art
Liquid crystal display devices most widely used at present are those utilizing nematic phase, but they have a drawback such that a display of large capacity such as of 1,000.times.1,000 lines is hard to realize. For example, in ordinary twisted nematic (TN) type liquid crystal display devices, contrast decreases as the number of lines increases, so that it is nearly impossible in practice to produce a display device of large capacity such as of 1,000.times.1,000 lines, with a good appearance.
In order to modify the drawback of the TN type liquid crystal display device, liquid crystal display devices of super twisted nematic (STN) type and double super twisted nematic (DSTN) type have been developed. However, they also have a drawback such that contrast and response speed decrease as the number of lines increases. Therefore, it is presently the utmost level that such display devices be provided with a display capacity of approximately 1,000.times.800 lines, by utilizing the upper-lower division driving method on a duty of 1/400.
On the other hand, there has been also developed a liquid crystal display device of active matrix type where thin film transistors (TFT) are arranged on a substrate, whereby a large capacity display of 1,000.times.1,000 lines and the like can be realized technically. However, the device has a drawback such that its production process takes a longer time with poor yield, leading to a very high production cost.
As a means to solve the problems described above, a ferroelectric liquid crystal display device, which is proposed as a liquid crystal display based on a principle different from the principle for the TN type display device, has been expected to be promising (see N. A. Clark, et al, Appl. Phys. Lett., 36, 899(1980)). In a such display device, a ferroelectric liquid crystal capable of exhibition chiral smectic C phase, chiral smectic I phase, etc. is utilized. Because the principle of the device is classified in a type utilizing memory characteristics, a larger display capacity may potentially be realized if response speed improves. Because the process does not require an active element such as thin film transistor, the cost for producing the liquid crystal device may not rise. Furthermore, the ferroelectric liquid crystal device has also a merit of a wide view angle, so that it is considered promising as an device for a display of a larger capacity of 1,000.times.1,000 lines or more.
The requirements for liquid crystal materials used in a ferroelectric liquid crystal device utilizing the smectic C phase aforementioned include the following items:
(1) exhibition of smectic C phase within a wide temperature range of which center is around room temperature; PA0 (2) chemical stability; PA0 (3) possessing large spontaneous polarization; PA0 (4) lower viscosity; PA0 (5) possessing phase sequences such as IAC (Isotropic-Smectic A-Smectic C) or INAC (Isotropic-Nematic-Smectic A-Smectic C); PA0 (6) a long spiral pitch of nematic phase; PA0 (7) a long pitch of smectic C phase; PA0 (8) possessing a large tilt angle.
Items (3) and (4) are required in order to obtain high speed response necessary for realizing a large capacity display, while items (5), (6) and (7) are required in order to obtain good orientation and item (8) is needed in order to improve contrast and brightness in liquid crystal display.
However, it is impossible at present to satisfy all of such requirements with a single compound. Therefore, a plurality of compounds are mixed together and practically applied as a liquid crystal composition. For producing a ferroelectric liquid crystal composition, it is generally employed the process comprising adding an optically active compound capable of inducing a large degree of spontaneous polarization in a non-chiral liquid crystal compound or composition exhibiting smectic C phase. In the process there may be used sometimes an optically active compound with no characteristics as liquid crystal.
As the non-chiral compound or composition exhibiting smectic C phase, there may be used a liquid crystal composition comprising a mixture of several kinds of pyrimidine compounds [see Hiroyuki Onishi, Tsuyoshi Kamimura, Hisahide Wakita, Shuko Oniwa, Isao Ota, National Technical Report, 33(1), 35(1987)]. However, it is a disadvantage for such process that a sufficiently wide temperature range cannot be attained in smectic C phase if only pyrimidine compound is used.
Thus, there has been used a process comprising preparing a liquid crystal composition with a wide temperature range of smectic C phase by using pyrimidine compound and ester compound, and adding an optically active compounds to the composition [see H. Inoue, A. Mizutome, S. Yoshihara, J. Kanbe, S. Iijima, International Display Research Conference, Post. Deadline Paper, (1988)]. On the other hand, some reports suggest that ester compound is inferior to pyrimidine compound, regarding response characteristics [see for example, Hiroyuki Onishi, Tsuyoshi Kamimura, Hisahide Wakita, Shuko Oniwa, Isao Ota, National Technical Report, 33(1), 35(1987)].